1. Field of the Invention
This invention relates to a chemical vapor deposition apparatus and a method of manufacturing annealed films, and more particularly to a chemical vapor deposition apparatus and a method of manufacturing annealed films with which a film is deposited on the surface of a semiconductor substrate, which is free from the deposition of an oxide film and the adsorption of impurities.
2. Description of the Related Art
Upon manufacturing a semiconductor device, a polysilicon film, an oxide film, a nitride film or the like which is an element of the semiconductor device is formed on semiconductor substrate by means of a heat treatment apparatus, a chemical vapor deposition apparatus, or the like. In this process, the degree of cleaning of the surface of the semiconductor substrate much affects the characteristics and the reliability of the semiconductor device as a product.
When, for example, a small signal transistor is manufactured, the portion of the substrate in which an emitter region is to be formed is exposed after a base region has been formed, and a polysilicon film is deposited. Then, impurity ions are implanted in the polysilicon film, and an emitter region is formed by diffusing the impurities in a semiconductor substrate by a heat treatment. The temperature, at which a polysilicon film is deposited on a substrate in a reduced pressure chemical vapor deposition apparatus, is around 600.degree. C. In the prior art, therefore, the temperature of the interior of the chemical vapor deposition apparatus is maintained at 600.degree. C. when semiconductor substrates are put in, handled in and taken out of the apparatus. The substrate is exposed to air in the outer atmosphere at a high temperature, and an oxide film of about 0.001 to 0.002 .mu.m is formed on the surface of the substrate. When, therefore, impurity ions are intended to be implanted in the semiconductor substrate by heat treatment, the diffusion of the impurities is suppressed by the oxide film. In consequence, a diffusion layer having a required impurity concentration cannot be obtained and an emitter resistance is increased as well.
In case when a film is formed on a semiconductor substrate, various wafer cleaning processes including physical and chemical ones are used to clean the substrate surface, and careful attention is paid so as not to contaminate the wafers with foreign matter when they are moved from a process to another process.
When, for example, semiconductor substrates are put in a heat treating apparatus and they are taken out of a chemical vapor deposition apparatus (hereinafter referred to as the "CVD apparatus"), an oxide film is sometimes deposited on each substrate. In the conventional heat treatment apparatus which prevents the build-up of the oxide layer, its inlet/outlet portion of semiconductor substrates is isolated from the outer atmosphere. After the atmosphere of the inlet/outlet portion has been evacuated, the substrates are entered in and taken out of the heat treatment apparatus, whereby an oxide layer (a natural oxide layer or the like) which otherwise might be formed by air from the outer atmosphere is prevented from being formed on the semiconductor substrate.
In this way, the prior art heat treatment apparatus can suppress the formation of the oxide film on the semiconductor substrate due to the atmospheric air, but water or the like which has already been adsorbed in the semiconductor substrate cannot be removed. Nor can be removed an oxide layer and/or impurities deposited on the semiconductor substrate during the time interval between the time the remaining oxide layer has been taken away and the time the semiconductor substrate is put in the heat treatment apparatus, and/or impurities such as fluorine adsorbed on the substrate surface.
Upon forming a new film on the substrate of a semiconductor, various means are used to clean the substrate surface to remove, for example, a remaining oxide film, such as a natural oxide film, and impurities existing on the substrate before the new film is formed. Such impurities deteriorate the characteristics and reliability of the semiconductor device.